Different types of electrolytes are widely being used in electrochemical devices, such as lithium secondary batteries, electrolytic condensers, electric double layer capacitors, electrochromic display devices, as well as dye-sensitized solar cells of which various studies are being conducted for future commercialization, and so the importance of electrolytes is growing day by day.
Particularly, lithium secondary batteries are attracting the most attention due to their high energy density and long cycle life. Generally, a lithium secondary battery includes an anode made from a carbon material or a lithium metal alloy, a cathode made from lithium metal oxide, and an electrolyte in which a lithium salt is dissolved in an organic solution.
Initially, lithium metal was used as an anode active material comprising an anode of a lithium secondary battery. However, because lithium has drawbacks of low reversibility and safety, currently, carbon material is mainly used as an anode active material of a lithium secondary battery. A carbon material has a lower capacity than a lithium metal, but has merits of a small volume change, excellent reversibility, and a low cost advantage.
However, as the use of lithium secondary batteries are expanding, the demand for high-capacity lithium secondary batteries is also increasing. Accordingly, there is a demand for high capacity anode active materials that may substitute the carbon material having low capacity. To meet the necessity, attempts have been made to use, as an anode active material, a metal that exhibits a higher charge/discharge capacity than a carbon material and allows electrochemical alloying with lithium, for example, Si, Sn, and the like.
However, this metal-based anode active material has a very great volume change during charging and discharging, which may cause cracks to an active material layer. Accordingly, secondary batteries using this metal based anode active material may suddenly deteriorate in capacity and reduce in cycle life over repeated cycles of charging/discharging, and thus are not suitable for commercial use.
To solve these problems, studies have been conducted to use an alloy of Si and/or Sn and another metal as an anode active material. However, the use of such an alloy contributes to the improvement of cycle life characteristics and prevention of volume expansion to some extent when compared with the use of metal alone as an anode active material, but the volume expansion generated during alloying with lithium still causes stress, thereby leading to an insufficient commercial use of secondary batteries.